In accordance with recent technical developments, various types of personal computers (PCS), such as desktop, tower and notebook types, have been developed and are popular. Lately, the application fields for PCS have been expanded.
For example, when a PC is equipped with a modem, the PC can be connected to a public telephone line. When a communication adaptor card (a TOKENRING.TM. card or an ETHERNET.TM. card) is installed into a PC, the PC can be connected to a local area network (LAN). Furthermore, since a LAN can be connected to the Internet via a router, recently, the PCs tend to be installed in network environments for use as information communication terminals.
The exchange of messages, such as the exchange of "e-mail", is the primary function of a PC in a network environment. An e-mail service system ensures the transfer of data and messages, and for international use, the charges are lower than those for telexes. Since only a person having a designated address (ID) can know of the existence of a message and its contents, e-mail can serve as a private mail service.
The types and forms of data communication terminals vary. For example, in addition to the conventional telephones connected to public telephone lines, various types of radio communication terminals that do not require cable connections have appeared. Radio communication terminals in current use are, for example, portable digital telephones (so-called cellular phones), portable analog telephones, PHS (Personal Handyphone System) phones, and pagers. However, the performances and features, i.e., the communication capabilities, of these terminals differ. For example, although the reception areas for PHS phones are smaller than those for portable telephones, no restrictions are placed on the allocation of frequencies for PHS phones, for which, as a result, purchase prices and communication fees are low. Both portable telephones and PHS phones, however, permit users to employ not only voice communication but also to receive transmissions of several Kilobytes of text data at a time. Whereas pagers, compared to these other radio communication terminals have considerably larger reception areas, have no transmission functions and can accept only about ten bytes of data at a time, and have, therefore, rather inferior data processing capabilities. Thus, by taking the differences in the communication capabilities into account, it can be understood that a single user may own and carry a number of different radio communication terminals and use them in accordance with the location.
Conventionally, e-mail systems have been employed as data exchange systems for transferring data between PCs installed in offices. Lately, however, the need to send messages to communication terminals in mobile environments has increased; as when, for example, a worker (using a PC) resident in an office must send an instruction or a message to another worker (carrying a PHS phone or a pager) in a mobile environment (unspecified location, such as outdoors).
If a worker in a mobile environment has only one reception terminal (e.g., a PHS phone) and does not respond to a PHS call because he or she is outside a reception area, no alternative exists to the abandonment of the data transmission effort. If, however, a worker in a mobile environment has a number of different radio communication terminals, in case the sending of a message using PHS is not possible, a natural reaction would be to try to send a message using a pager which has a larger reception area than is available with PHS.
A sender in an office will first try to send a message using PHS, for which the communication fee is lower than it is for a cellular phone but which has the same data processing capabilities; and then, if a communication link can not be established using PHS, the sender naturally will try to place the call using a cellular phone which in addition to having a larger reception area also has a higher communication fee, or a pager which has a larger reception area but which provides inferior data processing capabilities. Sending processing such as this is inefficient when it is allocated as a task to a user and is performed manually. In addition, receiving terminals have different data processing capabilities, such as permissible data size or character volume. Additionally, when the receiving terminal is switched from a PHS phone to a pager, the original message data is not always transferred seamlessly.